塩原層群, 宮島層の含珪藻薄層理泥岩について
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概要
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The Shiobara Paleo-lake deposits are situated in the northern margin of the mountainous region occupied by the Takahara and Shiobara volcanoes. Since A. G. Nathorst's report in 1883, many investigators have studied the plant fossils, geology and geomorphology of the region. The writer has studied the stratigraphy of the deposits, and established the stratigraphic classification shown in Table 1. Each formation of the Shiobara Group, except the Nakashiobara volcanic breccia and lava, is composed of two sedimentary facies, namely, the siltstone facies or the off shore deposits and gravel facies or the nearshore deposits. The siltstone facies of the Miyajima formation, distributed in the central area of the Paleo-Shiobara-lake near Myojin and Miyajima, comprises an alternation of gray coarse grained sandstone, grayish brown siltstone and thin laminated siltstone. The types of sedimentation cycles of the Miyajima formation are shown in Fig. 3. Laminated siltstones represent the uppermost part of each cycle, and are divided into two forms of laminations from their lithic characters. One form is composed of fine grained clastic sediments 1-10 mm. thick, and the other is the white diatom lamina, chiefly composed of diatom frustules, 1-1.5mm. thick. The latter is distributed particularly in the central area of the siltstone facies of the Miyajima formation near Miyajima. Diatom lamina are composed of an alternation of a set of lamina, and a set composed of the basal whitish gray diatom laminae 0.3 mm. thick, and the upper white diatom laminae 1-1.5mm. thick. Relative abundance of fossil diatom assemblages in the diatom lamina from the samples collected at Shiozawa of the Miyajima formation, are shown in Figs. 4 and 5. Whitish gray lamina are characterized with Stephawodiscus niagarae Ehrenberg, Cyclotella ocellata Pantocsek, Syleedra ulna (Nitzsch) Ehrenberg, and Melosira granulata var. angustissima Muller assemblages, and the numbers of species contained in the lamina are 15-20 species. White diatom lamina are chiefly composed of Stephanodiscus niagarae Ehr. and have but small quantity of Cyclotella ocellata Pant. and Synedra ulna (Nitz.) Ehr. The number of species in the white lamina are 4-10. Melosira granulata var. angustissima Mull. is abundant in the gray diatom lamina but absent in the white diatom lamina. Gray siltstones, intercalated in the diatom lamina contain diatom assemblages as gray diatom lamina, but the numbers of frustules contained in them amount to only 1/4-1/5 of the diatom lamina. The mode of formation of the alternations of diatom lamina may be related to the nutrient material dissolved in the lake water when the lamina were deposited.
- 東北大学の論文
- 1960-05-05